Altazimuth long exposure, how improve tracking

Hello everyone, now I'm several years that I make photograph long exposure in azimuthal mode with derotator field, with very satisfactory results. In all this time I made many tests, reaching the conclusion that the best results are obtainable only by using a OAG and an adaptive optics and we can not use a guide scope parallel. The only drawback is that the adaptive optics triggers the bump mount frequently, because of the bad tracking "passive (unguided)" of the LX200. Even if now I get good pictures, they are convinced that greater precision of tracking "passive" will further improve the sharpness of the images, bringing the values of RMS Wander AO-7 to 0.1, steadily, for each axis (currently these values vary from 0.1 to 0.4 with an average of 0.2 out of ten minutes of single exposure). Hence my intention to try something new to get this improvement. The new test will be set on the use of a double guide to further improve general tracking. In fact, I have many accessories, therefore I can try a test using two de-rotators field as follows:1) Use a Meade ED80 piggyback to LX200, using the Meade # 1220 de-rotator mounted behind the ED 80 and a Meade DSI III Pro for autoguiding the mount, all controlled via PHD and GPUSB connected to autoguider port on the telescope. 2) LX200> Pyxis 3"> AO-7> ST2000XM as main train camera, all controlled via MaximDL. The tilt range rate of the AO-7 will be set to 50%, disabling in fact the bump of the mountain. With this configuration, the LX200 is physically and steadily guided by the DSI, dramatically lowering the errors of RA and DEC axes triggered by passive tracking (without guiding), and the image of the guide telescope will rotate together with the main image thanks to the second de-rotator field. Furthermore, "the main image" is guided and stabilized by the AO-7, which will be less stressed thanks to more accurate tracking mount provided by DSI autoguiding. Clearly, the field of view from the guide scope and the main telescope must have the same center. This should work in theory, we'll see if it will work really well on the field.I will keep you informed on the progress of this test.Marco

Hi Chris, your observations are correct and theoretically in line with the extreme difficulty of work with 2 de-rotator field together. Anyway here are the facts: This evening I made the first test with results very, very interesting. First I aligned perfectly the two telescopes with 2 optical trains mounted. I had some difficulties to balance everything and in the end I had to settle for a balance that is not quite perfect. Despite this, I continued in the test going to NGC6503 as a subject because I have several data as a reference. First I calibrated PHD with DSI, after which I found the guide star in the AO-7 and then activate the calibration routine only adaptive optics. After performing the calibration, I activated 2 de-rotators field and I started driving the mount via PHD and also activating the AO-7. Everything worked very well and PHD drove perfectly the mount for 2 hours. During this time, the adaptive optics has corrected using a 20% max of the tilt of the mirror. However I have not noticed improvements in the RMS with respect to the bump via the AO-7. I have to say that tonight the seeing was not the best and the not perfect balancing may have affected the outcome. Also I have yet to work on totally new driving parameters. As a first test I am very happy for two reasons: The first is that, against the theory, everything worked wonderfully well. The second reason is that I realized that I could use the ED80 as the main telescope for extended objects and guiding with the LX200, all in azimuthal (fantastic). However, I will continue to do tests with a better balance and different driving parameters. For now I am very satisfied with the result. I enclose a picture of the setup used. Marco

Marco,I'm fascinated that you can make this work well but I can't help asking why? Maybe I'm missing something but your telescope doesn't appear big enough to preclude using a simple equatorial mount. Are you just trying to make it work or are you preparing to build something really big where an alt-az has significant mechanical advantages? Don't take me wrong...this is certainly impressive.John

Hi John,No, I do not have any plans for a larger telescope in the altazimuth. As I have explained in other posts the reason is simple. A LX200 12 "or larger is much easier to mount and unmount all times in altazimuth mode and even much more stable, respect to polar configuration with wedge (I do not have a permanent observatory) . Moreover there is no need to do the polar alignment and drift. I keep all the accessories mounted on the telescope (what you see in the picture), so when I start the photo session i just have to put the telescope on a tripod, connect the cables and so I'm ready to shoot in less than 15 minutes. This is really priceless for me. Surely is not the cheapest solution, but this is a secondary factor for me, because my priority is to have a good size of telescope and a good focal length lens (I love photographing small galaxies), easy and quick to make operational and especially that it works perfectly.Marco

Hi Chris, I've thought a lot at your reasoning on the theory that the guide scope should turn around the main mirror, so emulating a giant off-axis guider. Hence my question: Why the test with 2 de-rotator field has worked?I think because in reality the optical train of the main image has worked exactly like an OAG, relegating the guidance of the DSI for the sole purpose of keeping the guiding star of the AO-7 in the range of action of his mirror. In fact, the main image was de-rotated together with the reference star in the AO-7. Even in the traditional configuration, LX200 tracking ago without an autoguide and without a second de-rotator field, leaving the task at AO-7 of re-center the guide star with bump mount, and all of this works very well. In my tests the second de-rotator was used exclusively to keep synchronized the two images guiding (DSI and 237sensor SBIG), otherwise, even with DSI guider but without the second de-rotator on the ED80, the mirror AO-7 would have lost quickly her working reach. This is the conclusion I have arrived, but I'd like to know your opinion.Marco

Hi Chris, I've thought a lot at your reasoning on the theory that the guide scope should turn around the main mirror, so emulating a giant off-axis guider. Hence my question: Why the test with 2 de-rotator field has worked?
I think because in reality the optical train of the main image has worked exactly like an OAG, relegating the guidance of the DSI for the sole purpose of keeping the guiding star of the AO-7 in the range of action of his mirror. In fact, the main image was de-rotated together with the reference star in the AO-7. Even in the traditional configuration, LX200 tracking ago without an autoguide and without a second de-rotator field, leaving the task at AO-7 of re-center the guide star with bump mount, and all of this works very well. In my tests the second de-rotator was used exclusively to keep synchronized the two images guiding (DSI and 237sensor SBIG), otherwise, even with DSI guider but without the second de-rotator on the ED80, the mirror AO-7 would have lost quickly her working reach. This is the conclusion I have arrived, but I'd like to know your opinion.
Marco

Sort of hard to explain well without some diagrams.

Basically, I suspect that the angle between the main OTA and the guide OTA will change relative to the angle between the primary guide star and the imaging target. This should cause a smear who's magnitude will increase with the distance between the two OTA's, the OTA altitude and relationship to the celestial poles.

But hey, I could be all-wet on this without working through all of the math.

Very interesting, your suspicions might be justified. My test worked well for two reasons1) the two OTA have been perfectly aligned2) (this is very important) the guide star used with DSI (for coincidentally and fortunately) was perfectly in the center of the field of view of the camera.The question now is: if I had used a guiding star on one side or in a corner of the field of view by DSI, would worked equally well?This is definitely a point to clarify with the next tests.Marco

Hi Chris, it is difficult to explain these concepts in words, but I try. The tracking overall performed in the test, goes beyond the conventional way of understanding the guiding parallel as report: guiding/imaging = guide scope/main telescope. In reality what I tried is not autoguide intended as only autoguide with guide scope and imaging with conventional SCT telescope, because the main guiding (than the real), is dictated by the AO-7 that works radially with the main picture, therefore the second guide star (the real reference guide star) it's monitored by the second sensor by St2000XM, so radial . DSI serves only to maintain a costant tracking mount soft, but the real guider remains only the 237 guide sensor of ST2000XM that works in symbiosis with adaptive optics and main de-rotator field, therefore all together (all the optical train) rotates and autoguide works radially, so perfectly compatible with the theory.Another point is square and recalibrate autoguiding after a certain overall time tracking. In previous other posts I explained that MaximDL, with adaptive optics used in the conventional way (with bump mount), is able to maintain perfectly an offset over 75° of rotation image guide, providing more than 3 hours of consecutive exposures without having to square and recalibrate guider. This very important point, I'm not sure that PHD both able to do.Marco

Hi Chris, it is difficult to explain these concepts in words, but I try. The tracking overall performed in the test, goes beyond the conventional way of understanding the guiding parallel as report: guiding/imaging = guide scope/main telescope. In reality what I tried is not autoguide intended as only autoguide with guide scope and imaging with conventional SCT telescope, because the main guiding (than the real), is dictated by the AO-7 that works radially with the main picture, therefore the second guide star (the real reference guide star) it's monitored by the second sensor by St2000XM, so radial . DSI serves only to maintain a costant tracking mount soft, but the real guider remains only the 237 guide sensor of ST2000XM that works in symbiosis with adaptive optics and main de-rotator field, therefore all together (all the optical train) rotates and autoguide works radially, so perfectly compatible with the theory.
Another point is square and recalibrate autoguiding after a certain overall time tracking. In previous other posts I explained that MaximDL, with adaptive optics used in the conventional way (with bump mount), is able to maintain perfectly an offset over 75° of rotation image guide, providing more than 3 hours of consecutive exposures without having to square and recalibrate guider. This very important point, I'm not sure that PHD both able to do.
Marco

It sounds like you have created an incredibly-complicated setup. Why not just use a wedge and simplify your life considerably?

Having just gone through this thread, I could not resist answering the question above.

As I have explained in other posts the reason is simple. A LX200 12 "or larger is much easier to mount and unmount all times in altazimuth mode and even much more stable, respect to polar configuration with wedge (I do not have a permanent observatory) . Moreover there is no need to do the polar alignment and drift. I keep all the accessories mounted on the telescope (what you see in the picture), so when I start the photo session i just have to put the telescope on a tripod, connect the cables and so I'm ready to shoot in less than 15 minutes. This is really priceless for me. Surely is not the cheapest solution, but this is a secondary factor for me, because my priority is to have a good size of telescope and a good focal length lens (I love photographing small galaxies), easy and quick to make operational and especially that it works perfectly.
Marco

Chris, because this configuration, in fact, is much more simple, stable and fast to set up that with a wedge (I also have the wedge), so I know exactly the pros and cons of the two configurations. Azimuthal (also configured with a single de-rotator field, radial guide and adaptive optics) wins hands down compared with the wedge. The biggest flaw is that it is more expensive, but much, much more comfortable than a wedge, unless the telescope is mounted in a permanent observatory (then this is another matter entirely).

Nitegeezer, even I am amazed at how much weight can bring a LX200 in Altazimuth. On a wedge this is impossible.Marco

As I have explained in other posts the reason is simple. A LX200 12 "or larger is much easier to mount and unmount all times in altazimuth mode and even much more stable, respect to polar configuration with wedge (I do not have a permanent observatory) . Moreover there is no need to do the polar alignment and drift. I keep all the accessories mounted on the telescope (what you see in the picture), so when I start the photo session i just have to put the telescope on a tripod, connect the cables and so I'm ready to shoot in less than 15 minutes. This is really priceless for me. Surely is not the cheapest solution, but this is a secondary factor for me, because my priority is to have a good size of telescope and a good focal length lens (I love photographing small galaxies), easy and quick to make operational and especially that it works perfectly.
Marco

Getting four motors to dance in harmony is considerably-more challenging than just one RA motor. Now add in flexure, two guiders, mirror-flop, backlash and hysteresis.

Because I am at 19 degrees North Latitude, I use my 16" LX200GPS in Alt/Az with a derotator and a single OAG. That's three motors and a fair-amount of less complexity. But even then, a GEM mount would make for a much-simpler astrophotography setup.

Hi to all,
I completed my test of this particular configuration. Thanks to an evening with good seeing and a good balancing telescope, everything worked properly and tracking with 2 de-rotators field has been better than the classic solution with bump mount command through the AO-7. The difference is more pronounced with low frequency rate (2hz). Increasing the rate of AO guide (8/10 hz), the difference is there, but is less obvious.
This NGC6503 was obtained during the test and are 18 frames of 10 minutes (total time over 3 hours), without calibration (no dark, no bias, no flat). Very interesting the fact that all was obtained without recalibration autoguide between frames. The depth of the image is compromised by a high light pollution of my site, but the details are well etched and the stars are round and not too swollen (to be a SCT).
End of my tests, now I will devote my time only to take pictures.
Note: NGC6503 is an object far from simple, of small angular extension, and suitable to telescopes with high focal length.
Marco